Method and apparatus for evaluating a timeslot in a TDMA signal

ABSTRACT

A method and apparatus for evaluating interference within a timeslot of a received TDMA signal is described. An average received signal strength is calculated for a plurality of repeating segments within a TDMA timeslot. The segment sizes are dependent upon the efficiency of the error correction coding scheme utilized by a selected channel type. The calculated received signal strengths for the plurality of the segments are applied to a lowpass filter. The output of the lowpass filter is processed to determine the level of interference within the timeslot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/467,707 filed on Dec. 20, 1999 now U.S. Pat. No. 6,771,628, thedisclosure of which is incorporated herein by reference.

BACKGROUND

The present invention relates to interference measurements withintimeslots of a TDMA signal, and more particularly, to a method forevaluating a timeslot based upon an amount of interference within thetimeslot.

In order to insure the quality of voice communications over a cellularcommunications network, the system must avoid cellular calls from beingestablished on disturbed channels containing a level of interferencethat would degrade the quality of the calls to an unacceptable level.Interference in timeslots can be caused by co-channel interference fromanother transmitter sending on the same frequency but in another cell.The co-channel interferers are not necessarily slot-synchronized withthe TDMA transmitters in this cell. In order to evaluate the disturbedchannels, the interference level on selected channels must be measuredto enable an estimation of how much the interference level would affectthe quality of the call. The interference level on the downlink channelsis difficult for the base station to check, unless specified means areincluded in the standard. The interference level on the uplink channelscan be measured at the base station by monitoring received power levelsin idle timeslots, i.e., timeslots not used by any mobile connected tothat base station. This problem is more fully described in PCTapplication No. WO 97/31501, which is incorporated herein by reference.

Within analog systems, such as analog AMPS where one RF carrier isdedicated to one mobile station, a straightforward method for makingthis determination involves measuring and lowpass filtering the receivedsignal strength on idle channels for each analog channel. For TDMAsystems, the process is more complicated because interferencesupervision must be done for each timeslot. This problem is more fullyillustrated in FIG. 1 which illustrates an exemplary scenario where anuplink channel frequency is divided into three timeslots 5, 10, 15 ofwhich timeslot one 5 and timeslot two 10 are occupied by a first and asecond transmitting mobile stations respectively, connected to the basestation transceiver. Timeslot three 15 is idle and is disturbed by aco-channel interferer on the same frequency from a third transmitter inanother cell. It will be noted that the interference provided by theco-channel interferer actually occurs over timeslots two 10 and three15. This is because the second transmitter is not time synchronized/slotsynchronized with the timeslots of the receivers for the present basestation.

Existing methods of evaluating idle timeslots for e.g., determiningwhether or not co-channel interference should prevent the assignment ofa call to a timeslot involve determining an average interference levelfor the entire timeslot. Since the unsynchronized interference does notoccur over the entire time period of timeslot three 15, a determinationmay be made that the average interference level in timeslot three is lowenough to permit a new call to be set up on the timeslot, even thoughthe first portion of the timeslot is severely disturbed by theco-channel interference. This would present a serious problem forcertain types of connections since the channel protection (forward errorcorrection coding) may be weak, and the loss of even a few bits on theair interface may mean loss of the entire slot.

This problem arises because the average interference leveldeterminations are made over the entire timeslot period whilesignificant amounts of interference are only introduced in smallportions of the timeslot. The problem is accentuated if those smallportions contain critical information, such as bits used for errorcorrection. Thus, prior art system have difficulty handling cases whereunsynchronized strong interferers affect only portions of a timeslotsince the slotwise interference averages will not give a true picture ofthe quality of the mobile station to base station voice connectionprovided by the timeslot at all points within the timeslot. Thus, someway for measuring timeslot interference that reflects the true impact ofinterference throughout the timeslot is desired.

SUMMARY

The present invention overcomes the foregoing and other problems with amethod and apparatus for evaluating an idle timeslot within a TDMAsignal by detecting interference within the timeslot. The interferenceis detected by measuring the received signal strength of disturbingsignals. Initially, a channel type is selected and a segment or segmentsize is selected for a plurality of segment average calculations whichwill be performed throughout the TDMA timeslot. The size of the segmentis dependent upon the efficiency of the error correction coding schemeused by the TDMA signal of the selected channel type. Alternatively, theselected segment size may vary within the timeslot depending upon theimportance of information contained within particular portions of thetimeslot. For example, larger segment sizes may be utilized wherenon-important information will be transmitted at the beginning of atimeslot, and smaller segments utilized with the more importantfollowing information.

Within each of a plurality of segments defined by the selected segmentsize throughout the timeslot, an average received signal strength iscalculated. At least one of the calculated values of the averagereceived signal strength from the plurality of calculated averagereceived signal strengths is selected and input to a lowpass filter. Theoutput of the lowpass filter may be compared to a selected thresholdlevel to enable a determination of whether the idle timeslotinterference is low enough to enable a connection utilizing thetimeslot. If the filtered average received signal strength output by thelowpass filter exceeds a selected threshold value, an alternative slotmust be selected for the connection. If the selected threshold value isnot exceeded, the timeslot may be used for a call connection.Alternatively, the interference information may be stored for furtheranalysis e.g., for supporting network optimization.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be obtained by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 is an illustration of received power and co-channel interferenceon a particular frequency at a base station;

FIG. 2 is a flow diagram illustrating the method of the presentinvention;

FIGS. 3A–3D illustrate various embodiments of the segment described withrespect to FIG. 2; and

FIG. 4 is a functional block diagram of an apparatus for performing themethod of the present invention.

DETAILED DESCRIPTION

Referring now to the drawings, and more particular to FIG. 2, there isillustrated a flow diagram describing the method for detecting theinterference level within a particular timeslot and determining whetherthe interference level is high enough to render the timeslot unusablefor a call connection (i.e., a disturbed timeslot). Initially, a channeltype is selected at step 24 so that the modulation, channel coding anderror correction of the channel are known. Based upon the selectedchannel type, a segment size for performing a plurality of averagecalculations over the timeslot is selected at Step 25. The segment sizefor the average calculations is selected dependent upon the strength ofthe error correcting code used for the transmissions from the mobilestation to the base station. A strong channel coding scheme, forexample, IS136 ACELP, could use a larger segment since the errorcorrecting coding scheme may correct a large number of errors within areceived signal. A typical segment size, suitable for a slot containingACELP speech encoded and channel encoded information is 15 symbols(approximately 0.6 ms). A weaker channel coding scheme, for example,IS136 RLP1, would require the use of a smaller segment for the averagecalculations because less errors are able to be corrected by the codingscheme. Thus, the peak areas of interference throughout smaller portionsof the timeslot must be determined.

Next, an average of the received signal strength is calculated at Step30 for each segment defined by the selected segment size over the entiretimeslot period (in IS136 approximately 6.7 ms). Alternatively, theaverage can be calculated over a selected part of the timeslot period.The segments may be selected in a number of ways as illustrated in FIGS.3A–3D. In FIG. 3A, an average of the received signal strength (receivedpower) is determined for multiple separate segments 60 a–60 f of thetimeslot 65. Thus, for example, if the segment size 60 was 5 symbolslong, an average of received signal strength would be determined for thefirst 5 symbols 60 a (1–5), the second 5 symbols 60 b (6–10), the third5 symbols 60 c (11–15), and so forth until the end of the timeslot 65was reached. Alternatively, as illustrated in FIG. 3B, a sliding windowmay be used by progressively moving the segment 70 through the timeslot65 designating multiple overlapping segments. In this case, againassuming a 5 symbol segment 70, an average of the received signalstrength over symbols 1–5 (70 a) would be determined. Next an average ofthe received signal strength for symbols 2–6 (70 b), 3–7 (70 c), 4–8 (70d) and s forth would be determined until the end of the timeslot 65 wasreached.

Additional variations in segment sizes, illustrated in FIGS. 3C and 3D,may also be used in response to the type of coding scheme used withinthe received signal and the structure of the timeslot 65 (i.e., theselected channel type). For stronger coding schemes, the size of thesegment 75 may be increased as illustrated in FIG. 3C. Alternatively, asshown in FIG. 3D, different segment sizes 80 may be used in differentparts of the timeslot 65. For example, in some channel coding schemes,the first portion of the timeslot 65 may be less important than themiddle portion of the timeslot. Therefore, in the first portion of thetimeslot 65 a larger segment size 80 a, 80 b may be used sinceinterference within the first area does not create severe problems withthe call connection, while a segment size 80 c–80 f within the followingportions of the timeslot 65, containing the more important data, may besmaller to ensure detection of whether the interference level is toohigh, adversely affecting a call connection.

The determination of average signal strengths is repeated periodicallyand the resulting values are lowpass filtered for each segment. As willbe more fully described in FIG. 4, several average signal strengths forseveral segments are determined at one time. Each of the lowpassfiltered average signal strengths for each segment is then used toestimate an interference level at Step 40. This process may be carriedout in a number of methods and two particular embodiments are describedbelow with respect to FIG. 4. The estimated interference level islowpass filtered at Step 42. Alternatively, or in addition, theestimated interference level may be stored for further analysis, such asnetwork optimization.

At inquiry Step 45, the output of the lowpass filter is checked todetermine if the filtered average signal strength is above a preselectedlevel. The preselected level may be fixed or vary responsive to a numberof factors including, but not limited to, the capability of the mobilestation, the current load in the cell (higher congestion implies that ahigher level of interference may be accepted) and the modulationtechnique to be used. If the average signal strength does not exceed thepreselected level, the timeslot is not sufficiently disturbed to preventits use for a call connection, and the timeslot may be used at Step 50.If the filtered average signal strength exceeds the preselected level,an alternative timeslot must be selected at Step 55, and the aboveprocess is repeated to determine if the newly selected timeslot issatisfactory to support a call connection.

Referring now to FIG. 4, there is illustrated a block diagram of anapparatus for estimating the interference levels as part of the idleslot supervision method described in FIG. 2. The interference levelestimating apparatus 85 includes a sample buffer 90 which storesdigitized samples of the received signal strength within a plurality ofsegments 95. The sample buffer 90 contains segments 95 for one timeslot(6.67 ms) and is refilled with new data once every 20 ms. The varioussignal samples from the segments 95 are transmitted to average RSScalculation logic units 100 which determine an average received signalstrength for the samples contained within particular segments. A newcalculation takes place within the average RSS calculation logic units100 for each new update of the buffer 90, i.e., once every 20 ms. Thereis a separate average RSS calculation logic unit 100 for each segment 95within the sample buffer 90. The segments 95 within the sample buffer 90may be of different sizes or even overlapping as discussed earlier withrespect to FIG. 3.

Each average RSS calculation logic unit 100 provides an output valueeach 20 ms. This value is provided to a lowpass filter 110. The lowpassfilter 110 removes the fast random variations (greater than 0.5 Hz) inthe average received signal strength of the segments caused by fading(e.g., Rayleigh fading). Each lowpass filter 110 may be implemented as asimple first order lowpass filter with a time constant in the order of 1to 2 seconds. The output of the lowpass filter 110 is a slow varying(less than 0.5 Hz) signal strength value 115 for the associated segment95.

The slow varying signal strength value 115 for each segment 95 is inputto interference level estimator logic 120. The interference levelestimator logic 120 processes the outputs from each of the lowpassfilters 110 and outputs an estimated interference level 125. Theinterference level estimator 120 may be implemented in a number of ways.Two alternative methods are proposed below. However, it should berealized by one skilled in the art that the invention is not limited tothese particular implementations.

In a first alternative, a simple “peak find” method may be used. In thismethod, the outputs of the lowpass filters 110 are examined and acurrent maximum value S4 is determined according to the equation${S4} = {\underset{i = {1\ldots\mspace{11mu} N}}{MAX}( {\alpha_{i}\mspace{11mu}\bullet\mspace{11mu}{S3}_{i}} )}$where: N=number of segments the slot has been divided into;

S3_(i)=filtered RSS average for segment number i; and

α_(i)—weighting factor for segment number i. The weighting factor ischosen depending on the relative importance of the bits inside thatparticular segment for a selected channel type.

A second alternative works under the principle that any co- or adjacentchannel interference within the segment is likely caused by other TDMAtransmitters using bursts with the same length as the slot beingchecked. However, interferers are not slot synchronized with the checkedslot and will either interfere at the beginning or ending of the slot.Since it is not known how far into the slot, either from the beginningor from the end of the slot, the interference stretches, the followingequation may be used as an edge detection feature for determining howfar the interfering signals extends within the checked timeslot.Thereby, only the disturbed part of the timeslot may be included in thecalculations.${S4} = {{MAX}( {{\underset{n = {1\ldots\mspace{11mu} N}}{MAX}( \frac{\sum\limits_{i = 1}^{n}{\alpha_{i}{S3}_{i}}}{\sum\limits_{i = 1}^{n}\alpha_{i}} )},{\underset{N = {1\ldots\mspace{11mu} N}}{MAX}( \frac{\sum\limits_{i = 1}^{n}{\alpha_{N - i}{S3}_{N - i}}}{\sum\limits_{i = 1}^{n}\alpha_{N - i}} )}} )}$

If all α=1, e.g., if all segments are equally important, then thisequation can be simplified to:${S4} = {{MAX}( {{\underset{n = {1\ldots\mspace{11mu} N}}{MAX}( {\frac{1}{n}{\sum\limits_{i = 1}^{n}{S3}_{i}}} )},{\underset{N = {1\ldots\mspace{11mu} N}}{MAX}( {\frac{1}{n}{\sum\limits_{i = 1}^{n}{S3}_{N - i}}} )}} )}$which can be expanded into:S4=MAX(X ₀ , X ₁ , X ₂ , . . . , Y ₀ , Y ₁ , Y ₂, . . . )where; X₀ = S3₀$X_{1} = {{MAX}\{ {{S3}_{0},{\frac{1}{2}( {{S3}_{0} + {S3}_{1}} )}} \}}$$X_{2} = {{MAX}\{ {{S3}_{0},{\frac{1}{2}( {{S3}_{0} + {S3}_{1}} )},{\frac{1}{3}( {{S3}_{0} + {S3}_{1 +} + {S3}_{2}} )}} \}}$Y₀ = S3_(N)$Y_{1} = {{MAX}\{ {{S3}_{N},{\frac{1}{2}( {{S3}_{N} + {S3}_{N - 1}} )}} \}}$

The output of the interference level estimator logic 120 can be storedfor use as statistics in analyzing slot behavior, e.g., for supportingnetwork optimization work, or for decisions on whether a timeslot may beused for calls (unusable timeslots are temporarily “sealed”). The outputof the interference level estimator logic 120 will fluctuate due to slowfading and may cause oscillations in the system, e.g., slots jumpingback and forth from “sealed”. In order to prevent these oscillations ofslots from sealed to unsealed status, a lowpass filter 130 is connectedto the output of the interference level estimator logic 120. The timeconstant on the lowpass filter 130 is in the order of 10 seconds.

The response time for the entire interference estimation apparatus 85 isprovided by the time constants of the lowpass filters 110 and 130 withinthe apparatus. The time constants of lowpass filters 110 and 130 may beselected depending upon the type of traffic within a particular cell(i.e., stationary mobiles, slow moving mobiles, highway moving mobiles).Other filter types may also be used, e.g., higher order lowpass filters,prediction filters, etc.

If the timeslot can carry traffic with differing requirements forinterference levels, for example, within ACELP voice channels usingstrong channel coding or RLP1 encoded channels using weaker channelcoding, different segment sizes and/or time constants of lowpass filters110 and 130 may be needed for the different channel encoding schemes. Ina typical IS136 system, the channel coding scheme to be used for a call,depending on different factors such as mobile station capability, isonly known just prior to the call being setup. Thus, it is then too lateto start measuring the interference levels of the timeslots. In order toovercome this problem, interference level estimations may be performed,for example, for both an ACELP encoded call and a RLP1 encoded callusing a pair of interference level estimators 85. The estimators 85 willrun in parallel whenever a timeslot is idle. Just prior to call setupthe traffic control system (not shown) selects which output of anestimator 85 to use.

The previous description is of a preferred embodiment for implementingthe invention, and the scope of the invention should not necessarily belimited by this description. The scope of the present invention isinstead defined by the following claims.

1. A method for evaluating timeslot of a received TDMA signal byestimating an interference level within the timeslot, comprising thesteps of: calculating an average received signal strength for each of aplurality of repeating segments within the timeslot; estimating theinterference level based on at least one of said calculated averagereceived signal strengths, comparing the estimated interference level toa selected threshold level; establishing a connection on the timeslot ifthe interference level does not exceed the threshold level; andselecting another timeslot if the interference level exceeds thethreshold level.
 2. The method of claim 1 further comprising the step ofselecting a channel type for the timeslot of the received TDMA signal.3. The method of claim 2, further including the step of selecting a sizefor the plurality of repeating segments responsive to the selectedchannel type.
 4. The method of claim 2 further including the step ofselecting a plurality of sizes for the plurality of repeating segments,wherein the selected size of a segment is responsive to a position ofthe segment within the timeslot and the selected channel type.
 5. Themethod of claim 1 further including the step of lowpass filtering theestimated interference level.
 6. The method of claim 1 further includingthe step of lowpass filtering at least one of the calculated averagereceived signal strengths.
 7. The method of claim 1, wherein the step ofcalculating further comprises calculating a received signal strength fora plurality of overlapping, repeating segments.
 8. The method of claim1, wherein the step of calculating further comprises calculating areceived signal strength for a plurality of sequential, repeatingsegments.
 9. The method of claim 1, wherein the step of calculatingfurther comprises applying a weighting factor for each of the segments.10. The method of claim 1, wherein the step of selecting furthercomprises the steps of: determining a portion of the TDMA timeslotsubject to interference; and determining an average value for thecalculated received signal strengths within this portion.
 11. A methodfor selecting a timeslot of a received TDMA signal for a connection,comprising the steps of: selecting a segment size for a plurality ofsegment average calculations; calculating an average received signalstrength within each of a plurality of segments in the timeslot, saidsegments defined by the selected segment size; lowpass filtering each ofthe calculated average received signal strengths; estimating theinterference level based on at least one of said calculated and filteredaverage received signal strengths; lowpass filtering the estimatedinterference level; comparing the interference level to a selectedthreshold level; establishing a connection on the timeslot if theinterference level does not exceed the threshold level; and selectinganother timeslot, if the interference level exceeds the threshold level.12. The method of claim 11 further comprising the step of selecting achannel type for the timeslot of the received TDMA signal.
 13. Themethod of claim 11, wherein the step of calculating further comprisescalculating a received signal strength for a plurality of overlapping,repeating segments.
 14. The method of claim 11, wherein the step ofcalculating further comprises calculating a received signal strength fora plurality of sequential, repeating segments.
 15. The method of claim11, wherein the step of calculating further includes the step ofapplying a weighting factor to each of the segments.
 16. A method forestimating an interference level in a timeslot of a received TDMA signalfor a connection, the method comprising: dividing the timeslot into aplurality of segments; measuring a received signal strength in eachsegment to determine a signal strength measurement; saving the signalstrength measurement for each segment in a sample buffer; determining anaverage from the saved signal strength measurements in the sample bufferfor repeating timeslots; and comparing the average to a selectedthreshold level to estimate an interference level for the timeslot. 17.The method of claim 16, further comprising: establishing a connection onthe timeslot if the interference level does not exceed the thresholdlevel; and selecting another timeslot, if the interference level exceedsthe threshold level.
 18. The method of claim 16, further comprisinglowpass filtering the average to estimate the interference level for thetimeslot.
 19. The method of claim 16, further comprising determining apeak value for the timeslot.
 20. The method of claim 19, wherein thedetermining a peak value (S4) is determined according to an equation:${S4} = {\underset{i = {1\ldots\mspace{11mu} N}}{MAX}\mspace{14mu}( {\alpha_{i}*{S3}_{i}} )}$where: N=number of segments the timeslot has been divided into; S3_(i)=the RSS average for segment number i; and α_(i) =weighting factor forsegment number i.